In an effort to identify early genetic changes involved in the development of ovarian cancer, we constructed suppression subtraction hybridization (SSH) cDNA libraries from two early and two late stage ovarian tumors subtracted against normal ovarian epithelial cell brushings to identify aberrantly regulated genes in these tissues. These analyses led to the identification of HSulf-1, which encodes a novel 871 amino acid polypeptide containing a highly conserved sulfatase domain, as a gene that is downregulated in (75% of ovarian cancers. Similar downregulation is also observed in breast, pancreatic, renal cells and hepatocellular carcinoma lines. Additional data indicated that HSulf-1 encodes a cell surface polypeptide that exhibits sulfatase activity and diminishes the sulfation of heparin sulfate proteoglycans (HSPGs), specifically the N-acetylglucosamine residue of glycosaminoglycans. Emerging data suggesting that the sulfation state of HSPGs can influence signaling by heparin binding growth factors led us to hypothesize that HSulf-1 down-regulation modulates growth factor signaling in ovarian cancer. Consistent with this hypothesis, we observed that HSulf-1 restoration in HSulf-1-deficient ovarian cancer cells blunted signaling by fibroblast growth factor (FGF) and heparin-binding epidermal growth factor (HB-EGF). As a result, HSulf-1-transfected clones proliferated more slowly and were more sensitive to the induction of apoptosis by cisplatin and staurosporine than their HSulf-1-deficient counterparts. These observations suggest that HSulf-1 down-regulation contributes to the dysregulation of proliferation and apoptosis observed in ovarian cancer. Despite extensive surgery and systemic chemotherapy, usually with a taxane and a platinating agent, the vast majority of patients who present with disseminated (stage Ill or greater) ovarian cancer die of their disease. This observation highlights the need for improved understanding of drug resistance in ovarian cancer. To further evaluate the biological effects of HSulf-1 downregulation in ovarian cancer, we now propose to: 1) explore the mechanistic basis for the ability of HSulf-1 to modulate apoptosis; 2) determine the genetic basis of HSulf-1 downregulation; and 3) assess whether HSulf-1 downregulation results in resistance to other agents commonly used to treat ovarian cancer, including paclitaxel, topotecan, gemcitabine and doxorubicin, in vitro and in vivo and 4) evaluate the relationship between HSulf-1 expression and drug sensitivity in the clinical setting.